1. Field of the Invention
The present invention relates to an acceleration sensor for use in detecting acceleration in, for example, an air bag for a vehicle, and more particularly, to an acceleration sensor having an improved mounting structure for an acceleration detecting device.
2. Description of the Related Art
In a conventional vehicular air bag, an acceleration sensor is used to put the air bag into action in case of a collision. In other words, the acceleration acting at the collision is detected by the acceleration sensor, and the air bag is operated in response to output signals from the acceleration sensor.
Widely used as the acceleration sensor is a piezoelectric sensor which is excellent in mechanical shock resistance and capable of easily outputting the acceleration as electric signals. FIG. 6 is a plan view of an example of the conventional acceleration sensor, and FIG. 7 is a sectional view taken along the line B--B in FIG. 6.
An acceleration sensor 1 is formed of a case which consists of a case substrate 2 and a cover 3 having an opening at the bottom thereof, that is, the cover 3 is put on the case substrate 2 and fixed. An acceleration detecting device 4 is placed in a space enclosed by the cover 3. The acceleration detecting device 4 comprises a support member 4a fixed on the upper surface of the case substrate 2, and a piezoelectric acceleration detecting portion 4b attached to the support member 4a at one end in a cantilevered manner.
On the side of the acceleration detecting device 4, a signal circuit is located on a circuit substrate 5 to electrically process output signals taken from the acceleration detecting device 4.
The acceleration sensor 1 is also provided with lead terminals 6a to 6e drawn downward from the case substrate 2, and mounted for use by inserting the lead terminals 6a to 6e into a mounting substrate in a circuit portion of the air bag.
In the acceleration sensor 1 shown in FIGS. 6 and 7, however, the circuit substrate 5 is placed on the side of the acceleration detecting device 4, and the acceleration detecting device 4 and the circuit substrate 5 are dispersed on the case substrate 2 in the horizontal direction. Consequently, a large amount of space is required to mount the acceleration sensor 1 on the mounting substrate.
There also have been strong demands in applied products of the acceleration sensor, such as a vehicular air bag, for reducing the mounting space of the acceleration sensor.